A 55-million-year-old fossil of a mouse-sized primate has been identified as a crucial evolutionary link in the chain that led to apes and humans.

Four inches long, with a 5-inch tail and protruding eyes, Archicebus achilles probably thrived for millions of years during a warm period of Earth’s history, feasting on insects and leaping around in canopies of trees that surrounded a tropical lake in what now is China, according to a report published online Wednesday by the journal Nature.

“It was probably kind of a frenetic animal,” said study leader Chris Beard, a paleontologist at the Carnegie Museum of Natural History in Pittsburgh. “You could even think anxious — an animal that moves around a lot, very active, searching for its next meal, very agile in the trees, climbing and leaping around in the canopy.”

The remarkably complete fossil of Archicebus — derived from Latin and Greek for “ancient monkey” — helps make the case that primates first arose in Asia, even though the lineage leading to man later flourished and diversified in Africa.

“If we go along a tree to the point where all the primates began to evolve, they all point to Asia,” said paleontologist Xijun Ni of the Chinese Academy of Sciences, who led the fieldwork in a fossil-rich area of Hubei province better known for its spectacular fish fossils.